Re: With 4 disks should I go for RAID 5 or RAID 10 - Mailing list pgsql-performance

In response to Mark Mielke <mark@mark.mielke.cc>:

> Bill Moran wrote:
> >
> >> What do you mean "heard of"? Which raid system do you know of that reads
> >> all drives for RAID 1?
> >>
> >
> > I'm fairly sure that FreeBSD's GEOM does.  Of course, it couldn't be doing
> > consistency checking at that point.
> >
> According to this:
>
> http://www.freebsd.org/cgi/man.cgi?query=gmirror&apropos=0&sektion=8&manpath=FreeBSD+6-current&format=html
>
> There is a -b (balance) option that seems pretty clear that it does not
> read from all drives if it does not have to:

From where did you draw that conclusion?  Note that the "split" algorithm
(which is the default) divides requests up among multiple drives.  I'm
unclear as to how you reached a conclusion opposite of what the man page
says -- did you test and find it not to work?

>
>     Create a mirror.
>                  The order of components is important,
>                  because a component's priority is based on its position
>                  (starting from 0).  The component with the biggest priority
>                  is used by the prefer balance algorithm and is also used as a
>                  master component when resynchronization is needed, e.g. after
>                  a power failure when the device was open for writing.
>
>     Additional options include:
>
>                  *-b* /balance/  Specifies balance algorithm to use, one of:
>
>                              *load*         Read from the component with the
>                                           lowest load.
>
>                              *prefer*       Read from the component with the
>                                           biggest priority.
>
>                              *round-robin*  Use round-robin algorithm when
>                                           choosing component to read.
>
>                              *split*        Split read requests, which are big-
>                                           ger than or equal to slice size on N
>                                           pieces, where N is the number of
>                                           active components.  This is the
>                                           default balance algorithm.
>
>
>
> Cheers,
> mark
>
> --
> Mark Mielke <mark@mielke.cc>
>
>
>
>
>
>
>
>


--
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/

wmoran@collaborativefusion.com
Phone: 412-422-3463x4023

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